It is well known that an antenna is the key element to transmit/receive (transceive) microwaves in wireless technology such as wireless communication and wireless data transfer, where the antenna transforms electrical currents generated by a transmitter into microwaves and transmits the microwaves in free space. The antenna also captures microwaves and transforms them into electrical currents, which are then processed by a receiver.
There are two means to conduct signals between a transmitter/receiver (transceiver) and an antenna, and they are wire connecting and coupling feeding. Electrically connecting the transceiver and the antenna directly with an antenna signal feeding line is known as the wire connecting method, while electrically connecting the transceiver and an antenna-coupling element with the antenna signal feeding line is known as the coupling feeding method.
Further, in the method of coupling feeding the antenna is electrically connected neither to the antenna-coupling element nor the antenna signal feeding line. Since the antenna and the antenna-coupling element could be arranged in different and separate layers/positions, and sometimes further provided with a separating medium arranged in between, the coupling feeding method has more advantages than the method of wire connecting.
Such advantages includes the decrease of coupling effect between the antenna and the antenna signal feeding line (such effect is the cause of noises), the isolation of parasitic radiation from the feeding end, which electrically connects the antenna and the antenna signal feeding line, and the apply of the appropriate substrate matter to the antenna and the antenna signal feeding line respectively to increase the bandwidth of the antenna.
As stated previously, a microwave is basically the alteration between electric and magnetic field in free space. The presence of the antenna causes an alteration of the electric field, and the dimension of the antenna predetermined the alteration of the electric field and the microwave that it responds to. Take a dipole antenna and a PIFA antenna for example, the overall length of the dipole antenna is approximately half the wavelength of the microwave it responds to, while the overall length of the PIFA antenna is approximately one fourth of the wavelength the microwave it responds to.
Therefore, since an antenna with certain dimension used in for example an electronic device is to respond to certain microwave, the antenna is to transceive certain frequency of wireless signals, and that limits the use of both the electronic device and the antenna.